This invention relates to a slow start control network for a centrifuge and, in particular, to an electronic circuit for imparting an exponential acceleration characteristic to a centrifuge rotor.
Zonal separations by centrifugation are achieved by first preparing a liquid density gradient in a stationary vertically disposed container which has a length which exceeds its diameter. A sample to be separated is layered on the top of the gradient within the container. The container is then centrifuged and the particles within the sample migrate under the influence of the centrifugal force field until they are suspended in that portion of the gradient which corresponds to their density. In order to properly effect a density gradient zonal separation care must be exercised during the start-up sequence which brings the centrifuge rotor from a stationary position up to running speed. If the rotor start-up sequence is too abrupt the possibility exists that the sample will be disrupted or inverted. This occurrence would negate the separation.
One technique utilized to control the start-up sequence of a centrifuge so that the sample layer is not disrupted includes lowering the initial voltage applied across the drive motor windings to some predetermined low percentage (e.g., fifteen percent) of the rated applied voltage to effect a slow start. This may prove disadvantageous over a long term due to the tendency of an oxide coating to form over the windings. This coating eventually builds to a resistance level that is, in effect, impervious to the low voltage signal. Thus, start-up of the drive motor is prevented.
Another expedient is exemplified by U.S. Pat. No. 4,142,138 (Taylor et al.), assigned to the assignee of the present invention, wherein a dual spring arrangement is provided. The dual spring arrangement includes a first spring element which initially provides a first, lower, force to urge the motor brushes into contact with the commutator. Thereafter, as rotor speed increases, an increased force is applied by a second, larger, spring to urge the brushes more forcefully into contact with the commutator.
The drive motor control arrangement for a centrifuge includes, in the typical case, a power control device, as a silicon controlled rectifier (SCR), connected in series with the rotor drive motor. A capacitor controls the threshold voltage applied to the SCR gate electrode. The SCR is gated on at earlier points during each half cycle of line current as the start-up sequence proceeds by increasing the charging current flow to the capacitor. Capacitor current flow is regulated by a variable resistor. By charging the capacitor to the SCR threshold voltage earlier in a half cycle of line current more energy is applied to the drive motor windings thus increasing motor speed.
The variable resistor may take the form of an opto-isolator arrangement which includes a light emitting diode (LED) and an associated photo-resistor. The resistance of the photo-resistor decreases as the intensity of light emitted from the LED is increased. Light intensity from the LED can be caused to increase by increasing the current flow through the LED. The LED current is normally controlled by a regulating bipolar transistor connected in series therewith. In order to overcome system hysteresis a manually adjustable thresholding network is provided so that a predetermined base current is drawn through the regulating bipolar transistor immediately upon start-up. This increases the intensity of the light emitted from the LED so that the variable resistance is reduced. Rotor motion begins in a relatively short time after power application. However, this manual adjustment is sometimes difficult to make.
Accordingly, it would be advantageous to provide a slow start control network which overcomes system hysteresis without the necessity of a difficult manual adjustment. Further, it would be advantageous to provide a control network which, upon detection of the initial current flow through the LED sufficient to overcome system hysteresis, thereafter causes LED current flow to gradually increase, preferably at an exponential rate, until operating speed is reached.